Conventionally, optical information storage media, such as CD, CD-ROM, CD-R, DVD, PD, MO and MD, have been widely used for mass storage of audio signals or image signals. In particular, magneto-optical storage media and phase change storage media are attracting attention as rewritable high-density storage media, and research and development thereof is being increasingly conducted for higher density. Besides, research and development on the optical information storing devices that access information stored in such optical information storage media is being increasingly conducted.
In general, an optical information storage medium has the shape of a disc, a multi-helical track or a large number of concentric tracks are provided thereon in parallel with each other, and information is stored in the tracks in the form of optically readable marks. In order to increase the recording density of the optical information storage medium, there can be adopted one of two approaches, that is, shortening the track pitch or increasing the track recording density. Both of the approaches can be implemented if the wavelength of a semiconductor laser used for recording and reproduction is shortened. However, it will be take some time to provide an inexpensive semiconductor laser that continuously emits light of short wavelengths, such as blue or green light, stably at room temperature for a long time.
Under such circumstances, a technique of substantially increasing the track recording density using a semiconductor laser of a currently possible wavelength is being searched for. For example, concerning the magneto-optical storage media, there is proposed a technique of applying a reproduction magnetic field to limit the reproduction target area to an area smaller than the laser spot (referred to as magnetic super resolution).
On the other hand, as a technique of shortening the track pitch using a semiconductor laser of a currently possible wavelength, there is proposed a technique referred to as land and groove recording, in which a storage medium has grooves and lands alternately arranged, and both the grooves and the lands are used as tracks for recording information. The land and groove recording is expected to realize substantial shortening of the track pitch.
If the track pitch is substantially shortened, substantial improvement of recording density can be expected. However, there rises a worry that cross talk between adjacent tracks increases, and it is desirable to reduce cross talk.
As a recording method suitable for high density information recording, a mark edge recording method is known. In information recording according to the mark edge recording method, in general, RLL (run-length limited) modulation, referred to as RLL(1, 7), is performed on a bit data string representing the information, and a mark depending on the modulated bit data string is recorded on the optical information storage medium.
It is known that, according to such a recording method, if particular cyclic data is recorded, a mark pattern having repetition of long marks and short spaces or a mark pattern having repetition of short marks and long spaces occurs on the optical information storage medium (hereinafter, these mark patterns will be referred to as a worst mark pattern). In addition, it is also known that such worst mark patterns provide particularly intense cross talk. In order to reduce cross talk, there is a need for a technique of avoiding occurrence of such worst mark patterns.
As a technique of avoiding occurrence of the worst mark patterns described above, for example, there is proposed a technique of adding a pseudorandom number to a bit data string representing information to be recorded, thereby preventing occurrence of the particular cyclic data described above. However, this technique has a problem of high cost because it is essential that a bulk circuit that produces pseudorandom numbers is integrated into the optical information storing device. In addition, there is no practical technique of avoiding occurrence of the worst mark patterns described above other than the technique of using pseudorandom numbers.
Such a problem becomes significant when the land and groove recording method is adopted. Even if the land and groove recording method is not adopted, however, the same problem is generally encountered when the track pitch of an optical information storage medium is desirably shortened.